1. Field of the Invention
The present invention relates generally to the field of radar and concerns more particularly a method of processing the signals of a radar of the monopulse type, for estimating a parasite phase introduced between these signals by the ultrahigh frequency formation circuits of the sum and difference channels.
2. Description of the Prior Art
It is known that with a radar of the monopulse type the angular location of targets may be obtained by processing the different signals received simultaneously from these targets and corresponding to different beam directivities.
Thus, as shown very schematically in FIG. 1, by providing two sources respectively in the azimuth plane (1a, 2a) and in the elevational plane (1e, 2e), at reception there are available for each of these two planes two signals of which the sum .SIGMA. and the difference .DELTA. may be formed by ultrahigh frequency means (3, 4), which is tantamount to having an antenna with a beam corresponding to the .SIGMA. channel and another beam corresponding to the .DELTA. channel, and thus allows angular location of the targets to be obtained simply.
It is also known that, with the reradiation of a target being a sum of the waves reflected by each of the reflecting elements which form it, the signals S and D received respectively in the sum and difference channels, in azimuth or in elevation, have as complex representation: ##EQU1## with: s(.theta..sub.i): gain of the sum channel at the angle .theta..sub.i ;
d(.theta..sub.i): gain of the difference channel at the angle .theta..sub.i ; PA1 a.sub.i : amplitude relative to the ith reflector; PA1 .phi..sub.i : phase shift introduced by the ith reflector; PA1 d.sub.i : distance between the radar and the ith reflector; PA1 .PSI.: parasite phase introduced between the .SIGMA. and .DELTA. channels by the ultrahigh frequency formation circuits of the sum and difference channels.
It is also known that, in the particular case of the elevational plane, the parasite phase .PSI. between the sum and difference channels may be evaluated in the following way:
let ##EQU2## where D* designates the complex conjugate quantity of D, and where: ##EQU3##
If we calculate the mean value of P over different distances divisions of the radar, the phase ##EQU4## being considered as random, we have: ##EQU5## where E designates the expectation operator.
The term "a" is real, but is a priori of unknown sign because of the factors d(.theta..sub.i) which may be positive or negative. In fact, referring to FIG. 2 which shows the trend of the sum and difference diagrams, .SIGMA. and .DELTA., in the azimuth plane or in the elevational plane, the sum diagram has the maximum in the direction of the axis of the antenna, whereas the difference .DELTA. diagram has on the contrary a minimum in this direction and is, on each side of this direction, either in phase (.DELTA.+) or in phase opposition (.DELTA.-) with .SIGMA..
By averaging P, we can then a priori only estimate the phase .PSI. to with in the sign.
Now, it so happens that in the elevational plane this ambiguity of sign may be relatively simply removed for the sign of d(.theta..sub.i) is always related to the rank of the range bin considered being negative for the closest range bins and positive for the furthest range bins. By averaging P over the different range bins, an estimation of the phase .PSI. is obtained without ambiguity of sign after correction of the sign as a function of the considered range bin.